In triangle \( ABC \), the centroid \( G \) divides each median in a 2:1 ratio. Since \( D \), \( E \), and \( F \) are the midpoints of sides \( BC \), \( CA \), and \( AB \) respectively, the segments \( AD \), \( BE \), and \( CF \) are the medians.
Given that \( BG = 24 \) mm, and knowing that \( G \) is located 2/3 of the way along median \( BE \), this means that segment \( GE \) is the remaining 1/3 of the median.
Since \( BG \) is 24 mm (the portion from vertex \( B \) to the centroid), segment \( GE \) will be:
\[ GE = \frac{1}{2} \times BG = \frac{1}{2} \times 24 \text{ mm} = 12 \text{ mm} \]
Thus, the length of segment \( GE \) is:
\[ \boxed{12 \text{ mm}} \]